1. Field of the Invention
This invention relates to a process and apparatus for combustion in a surface combustor-fluid heater in which the combustion is carried out in stages within the pores of the stationary porous bed and heat transfer is achieved using heat exchange surfaces embedded in the stationary porous bed resulting in very high combustion intensity, very high heat transfer rates, improved energy utilization efficiency, ultra-low combustion emissions, and lower capital and operating costs. The use of staged combustion within a porous matrix surface combustor-heater in accordance with this invention reduces NO.sub.x formation by the combustion process to levels less than about 10 vppm.
2. Description of Prior Art
In general, heat energy may be transmitted by conduction, convection and/or radiation. Heat transmission by radiation and utilization of infrared energy has many advantages over conventional heat transmission by convection and conduction. The operation and construction of infrared burners and radiant heaters is relatively simple and, thus, more economical than other types of heat generation means. The intensity of radiant heat may be precisely controlled for greater efficiency and infrared energy may be focused, reflected, or polarized in accordance with the laws of optics. In addition, radiant heat is not ordinarily affected by air currents. One type of gas-fired infrared generator currently available is a surface combustion infrared burner having a radiating burner surface comprising a porous refractory. The combustion mixture is conveyed through the porous refractory and burns above the surface to heat the surface by conduction. One such burner is taught by U.S. Pat. No. 1,331,022. Other surface combustors are taught by U.S. Pat. Nos. 4,666,400; 4,605,369; 4,354,823; 3,188,366; 4,673,349; 3,833,338; and 4,597,734. See also U.S. Pat. No. 3,738,793 which teaches an illumination burner having a layered porous structure, the layered pores maintaining a stable flame in a thoria-ceria illumination burner in which combustion occurs not within the pores of the combustor, but rather on the surface of the top layer.
Control of combustion emissions, in particular NO.sub.x emissions, is an important requirement for surface combustors which are generally known to produce high combustion intensity and, thus, high combustion temperatures. It is generally known that to reduce NO.sub.x formation within the combustion process, it is necessary to simultaneously remove heat from the combustion process as combustion of the fuel occurs. U.S. Pat. No. 5,014,652 teaches a fluidized bed combustion reactor/fluidized bed cooler comprising a vertical reactor chamber designed to contain two separate fluidized beds, one of which contains cooling coils through which a cooling fluid is flowing to remove heat from the bed. U.S. Pat. No. 3,645,237 teaches a fluidized bed water heater in which water is heated or steam is produced by passing water through heating coils embedded in the fluidized bed. Similarly, U.S. Pat. No. 4,499,944, U.S. Pat. No. 4,779,574, and U.S. Pat. No. 4,646,637 teach a heat exchanger installed in a fluidized bed. And, U.S. Pat. No. 4,899,695 teaches a fluidized bed combustion reaction in which heat is transferred from the fluidized bed to water-containing tubes surrounding the reactor.
An alternative to control of NO.sub.x emissions by removal of heat from the combustion process as combustion of the fuel occurs is the use of staged combustion in which the fuel and/or oxidant are introduced into the combustion chamber in stages and under conditions which maintained the combustion temperature generally below the temperature required for substantial NO.sub.x formation. U.S. Pat. No. 5,080,577 and U.S. Pat. No. 5,160,254 both teach staged combustion within a porous matrix combustor. The '577 patent teaches a method and apparatus for low NO.sub.x combustion in which fuel and an oxidant mixture is burned in a first combustion zone under fuel-rich conditions and in a second combustion zone under fuel-lean conditions. The '254 patent teaches a porous matrix combustor with primary and secondary combustion zones, the primary combustion zone being fuel-lean and the secondary combustion zone being fuel-rich. To provide the desired fuel-rich conditions in the secondary combustion zone, fuel and oxidant are both injected into the porous matrix downstream of the primary combustion zone. See also Russian Patent SU 1393994.
U.S. Pat. No. 5,308,473 teaches a low NO.sub.x fluidized catalytic cracking regeneration process and apparatus which utilizes dilute phase afterburning to superheat catalyst entrained in the dilute phase region above the fluidized bed. Similarly, U.S. Pat. No. 5,288,397 teaches a process and apparatus for dense phase, at least partially co-current, fluidized bed regeneration of a fluidized catalytic cracker catalyst.
One problem associated with fluidized bed combustors is the amount of particulate matter generated by such beds which is carried out with the products of combustion exhausted by the combustor. In addition, the abrasiveness of the fluidized bed particles against the outer surfaces of heat exchangers disposed in the fluidized bed causes erosion of the heat exchanger surfaces. Finally, pressure drop of flow through the fluidized bed is high due to the high flow of velocity required for fluidization.